Small motor boats and dinghies are prone to capsize. In most cases, the swamped or capsized vessel has a degree of reserve buoyancy, meaning that the vessel does not sink per se. In the case of a conventional forward or tiller-steered open boat, these are generally of bond wood or cold moulded or strip planked wood, aluminium, fibreglass or other fibre reinforced resin material, or rotary moulded plastic (poly).
Boats may be inherently buoyant, such as some timber boats, foam or balsa cored resin boats or double skinned poly boats. Others may be provided with constructed buoyancy, such as watertight or foam filled chambers or voids. For example, it is commonplace to provide aluminium dinghies with thwarts of inverted, flat-bottomed U-shape, riveted or welded to the hull and/or transom inside the boat and having blocks of buoyancy foam bonded into the hollow undersides.
Self-righting devices for vessels comprising a gas-inflatable bladder arrangement are known. In general these require a purpose made frame or platform elevated above the vessel hull at the stern and having bolted thereto a relatively large inflatable chamber. For example a self-righting system may include a heavy-duty coated fabric chamber including sewn-in reinforced tabs having multiple eyelets or trapped-head threaded studs to permit bolting and/or lacing to the frame or platform. The volume of the chamber calculated by taking into account the weight, beam and load of the boat. The inflatable chamber is attached to the top of the frame at the stern of the boat and is connected to an external CO2 inflator bottle by hose and trigger assembly. In a capsize, the manually operated CO2 inflator inflates the chamber to self-right a suitable vessel, typically within half a minute.
Protection from the sun for occupants of a small vessel (and rain when not under way) is usually by way of a Bimini top. These structures comprise two or more generally stainless steel (or occasionally aluminium or composite) tubular bows each formed with port and starboard risers supporting an athwart overhead bow portion. One of the bows may be pivoted to opposed gunwale locations to be rotatable fore-and-aft. One or more other of the bows may be pivoted to opposed riser points on the first bow to also be rotatable fore-and-aft. The net effect is of an overhead framework that may be folded down to the gunwales. The bow portions are located in respective pockets of a poly canvas or vinyl Bimini cover. The assembly of the bows is strut, strap or cable stayed, with tension in the struts, straps or stays functioning to keep the Bimini cover taut between the bow portions.
For the most part such vessels are powered by outboard motors. However, some are inboard-engine powered, such as Hamilton-style jet boats and drive leg-driven boats.
To a greater or lesser extent, a small boat of the type described will, when swamped, reach a stable attitude floating in the water. Depending on the relative location of the centre of buoyant lift relative to the centre of gravity, the attitude may be upright or inverted, or bow up and bow down. The most common attitude for an outboard-powered dinghy having two or more buoyant thwarts is inverted and bow-high. In an accidental swamping and capsize, the buoyant vessel forms a poor support for occupants awaiting rescue, both from the point of view of exposure to the elements and predators and from the point of view to access to safety gear in the vessel. Clinging to an upturned hull in rough seas can prove extremely challenging. As fatigue and exposure take their toll the passengers find it more and more difficult to cling to a wet, slippery and very unstable object. Food, water, clothing, and all safety gear will be located inside the upturned vessel out of reach and possibly lost to the occupants.
It has been widely reported that approximately 85% of boating fatalities in boats under 6 m are the result of swamping or capsize. Capsize is often unexpected and happens in a few seconds. There is a likelihood of being unable to grab everything that is needed for enhancing odds of survival other than what is immediately to hand. Many persons have died within the few hours following capsize before help reached them. Men on a fishing trip are the most likely casualty.
For various reasons the aforementioned self-righting systems are unsuited to self-right a Bimini or Targa top-equipped vessel. At first instance the presence of a Bimini or Targa top effectively prevents a push pit or support frame of sufficient height to be positioned at the stern of the vessel. Secondly, if a frame or support were to vertically clear the Bimini or Targa, the excess weight set high would compromise roll stability.